Photographic process printers use a cathode ray tube (CRT) to project an image onto a photosensitive media. After the image is projected onto the media, the media is processed to develop the latent image. In order to transfer the image from the surface of the CRT to the surface of the media while maintaining image focus, an array of optical fibers are adapted to transfer the light from the surface of the CRT to the surface of the media.
The phosphors used to provide color output of a CRT may not match the color spectrum necessary to properly expose the print media. To better match the light spectrum output of the CRT to the light spectrum required by the print media, filters are needed between the phosphor of the CRT and the print media.
FIG. 2 shows a prior art method of using filters to optically match the spectrum output of a CRT to the spectrum requirements of a print media. FIG. 2 shows a CRT glass envelope 202 having a CRT face 204, which is the outside surface of the CRT glass envelope 202. An array of optical fibers 206 is built into the CRT 202 and is arranged to transfer the light from the CRT to the surface of a print media 208.
When constructing the CRT 202, filters 216 and 218 are first formed on the inside surface 220 of the CRT 202, which is also the inside surface of the array of optical fibers 206. These filters are typically dichroic filters vacuum deposited onto the inside surface 220 of the glass envelope 202. CRT phosphors 210 and 214 are then formed on top of the filters 216 and 218 such that the electron beam 224 emanating from and electron gun 222 will first strike the phosphor to create light, and then the light will be filtered by the filter before the light emits from the face 204 of the CRT.
FIG. 2 shows three phosphors 210, 212, and 214, with two filters 216 and 218 used to filter the output of two of the phosphors. The third phosphor 212 is formed directly on the inside surface 220 of the CRT, indicating that no filtering is needed to match the spectrum output of this phosphor to the spectrum needed to properly expose the print media 208 for the color produced by phosphor 212.
Forming the filters 216 and 218 on the inside surface 220 of the CRT creates significant problems. The filters must be formed onto the CRT at the beginning of the process of constructing the CRT, because the filters must be deposited on the inside surface of the CRT prior to the phosphors being formed on the inside surface of the CRT. This is necessary in order to allow the electron beam 224 emanating from the electron gun 222 in the rear of the CRT to first strike the phosphor in order to provide light. The light output of the phosphor must then be filtered before it is emitted through the front of the CRT. Since the filters must be formed first, they must be formed before the phosphor coating is known to be good, thus, causing yield problems in the CRT production.
Another problem with forming the filters on the inside surface 220 of the CRT is that the process used, vacuum deposition, is an expensive process.
It is thus apparent that there is a need in the art for an improved method or apparatus that allows filters to be placed on a CRT after the phosphor coating has been placed on the CRT, thus after the CRT is known to have good phosphor coatings. Another need in the art is for a method of creating the filters that does not use vacuum deposition. The present invention meets these and other needs in the art.